1. Field of the Invention
The present invention relates to a method for producing a reaction sintered ceramic improved in mechanical strength, the toughness, and thermal insulation.
2. Description of the Prior Art
Ceramics are high in the resistance to heat, shock, and wear as well as in mechanical strength and have widely been studied for use as materials of combustion chamber in internal combustion engine. As some having been marketed for practical use, the ceramics need to be high in thermal insulation or low in thermal conductivity for minimizing thermal loss of an applicable system. For use as structural materials, ceramics are required to have a minimum change in dimensions after a sintering step in the production method. To satisfy such requirements, the ceramics are commonly produced by the use of a reaction sintering process where the sintering is carried out under the atmosphere of nitrogen gas. This process allows a resultant ceramic to be small in contraction after the sintering and thus ensures a minimum change in the overall dimensions. There has been a variety of ceramics produced by the reaction sintering process and some of them will be explained below.
As disclosed in Japanese Patent Laid-open Publication 56-134568 (1981), a reaction sintered ceramic comprising a solid solution of silicon nitride (Si.sub.3 N.sub.4) and having a porosity of 5% to 15% is produced by converting a powder mixture of 80% or less by volume of silicon (Si), 10% to 85% by volume of alumina (Al.sub.2 O.sub.3), and 0% to 15% by volume of magnesium oxide (MgO) to a given shape, subjecting the shape to reaction sintering at 1200 to 1400.degree. C., and heating it to 1500 to 1800.degree. C. The resultant ceramic is increased in mechanical strength and toughness but not improved in thermal insulation.
Another ceramic having a high mechanical strength and toughness is disclosed in Japanese Patent Laid-open Publication 58-60676 (1983). For increasing its mechanical strength and toughness, ceramic of silicon nitride (Si.sub.3 N.sub.4) is produced by milling, mixing, shaping, and heating for reaction sintering a mixture of 60% to 98.9% by weight of silicon, 0.1% to 15% by weight of a sintering accelerator selected from chromium (Cr), chrome oxide (Cr.sub.2 O.sub.3), and chrome nitride (CrN), and 1% to 25% by weight of one or more components selected from oxide of a rare earth element, aluminum oxide, and zirconium oxide. It however fails to improve thermal insulation.
Disclosed in Japanese Patent Laid-open Publication 4-342470 (1992) is a further reaction sintered ceramic which is produced by mixing a parent material of silicon powder with nitrogen and oxygen, adding at least one element selected from titanium (Ti), zirconium (Zr), hafnium (Hf), yttrium (Y), boron (B), and aluminum (Al) to the parent material mixture, subjecting the mixture to reaction sintering under the atmosphere of nitrogen gas to form a nitride, heating the nitride in the atmosphere of oxygen gas for oxidation of a part of the nitride. The reaction sintered ceramic is increased in mechanical strength and the thermal insulation but tends to be reduced in volume after the sintering step. Hence, it will hardly be used as a member of a high precision machinery assembly.
Also, a ceramic is disclosed in Japanese Patent Laid-open Publication 1-226767 (1989), which is minimized in the dimensional change after the reaction sintering by bonding ceramic particles to each other with an electrically conductive nitride substance produced from a metal powder component of the mixture and thus reducing voids between the particles. The ceramic is however low in mechanical strength and high in thermal conductivity.
Furthermore, a ceramic reinforced with a long or short fabric is provided for increasing the toughness as disclosed in Japanese Patent Laid-open Publication 61-91063 or 61-197472 (1986). Such composite ceramic fails to attenuate the volumetric contraction after the sintering and to improve the thermal insulation.
It is an object of the present invention to eliminate the foregoing drawbacks of the prior art and particularly, to provide an improved method for producing a reaction sintered ceramic where:
(1) the mechanical strength or toughness is increased, PA1 (2) the thermal conductivity is minimized thus increasing the thermal insulation, and PA1 (3) the volumetric contraction after sintering is minimized to ensure a dimensional accuracy.